pn junction diode

PN Junction diode is a device made of two semiconductor material joined together with the required amount of impurity. These materials are N-type, having electrons are majority carrier and P-type material, having holes are majority carrier.

With proper biasing, PN diode allows current to flow in SINGLE DIRECTION.

A PN Junction Diode is one in all the only semiconductor devices around, and that has the characteristic of passing current solely|in just|in mere|in exactly|in precisely|in barely} one direction only.

pn junction diode

However, not like an electrical device, a diode doesn’t behave linearly with regard to the applied voltage because the diode has associate exponential current-voltage ( I-V ) relationship and thus we are able to not delineate its operation by merely victimisation associate equation like Ohm’s law.

Also Read: Zener Diode, Its Working, Zener Breakdown And Its Applications

If an appropriate positive voltage (forward bias) is applied between the 2 ends of the PN junction, it will provide free electrons and holes with the additional energy they need to cross the junction because the breadth of the depletion layer round the PN junction is shriveled.

Construction & Working Of PN Junction Diode-

Construction-

On a wafer of n-type silicon(usually silicon + phosphorus), an aluminium film is placed and then they are heated at a high temperature. This results in the diffusion of aluminium into silicon and thus our PN junction diode is ready to be used in various semiconductor devices. It can also be prepared by diffusing phosphorus into a p-type semiconductor.

Working of PN junction diode-

Let’s consider an unbiased diode(diode not connected to any voltage source).

pn junction diode

On seeing the diagram, we observe that the holes are the majority carriers on the left side(p-type) whereas electrons are the majority carriers on the right side(n-type).

Thus, to distribute the concentration over the diode, holes start diffusing towards the right whereas electrons diffuse towards the left. This generates a current flowing from left to right[p-type to n-type] (as direction of current is always OPPOSITE to the direction of flow of electrons and same as the direction of movement of holes). This current is known as DIFFUSION CURRENT.

Also Read: Zener Diode, Its Working, Zener Breakdown And Its Applications

Therefore,

Idf=Ie+IhIdf=Ie+Ih

i.e. Diffusion current = current due to flow of electrons + current due to movement of holes.

We know that both p-type and n-type semiconductors are neutral initially. Thus, the diffusion of holes and electrons causes an excess positive charge in the n-region and an excess negative charge in the p-region. Thus the system becomes UNSTABLE.

This double layer of charge creates an electric field(direction n-type to p-type[right to left])[because the direction of field is always away from positive charges(right) and towards negative charges(left)]. This field exerts a force on the electrons and holes, against their diffusion.

pn junction diode

Thus, a potential difference is created in the junction diode with n-region at a higher potential than the p-region. This potential is known as barrier potential. The junction region is now almost void of charge carriers and thus it is known as depletion layer.

Also Read: Zener Diode, Its Working, Zener Breakdown And Its Applications

When an electron-hole pair is created in the depletion region, the electron is pushed by the barrier electric field towards the n-side and the hole is pushed towards the p-side and this gives rise to a current from n-side to p-side(right to left). This current is known as DRIFT CURRENT and its direction is opposite to that of diffusion current.

pn junction diode

Thus for an unbiased diode,

Idf=IdrIdf=Idr, i.e. Inet=0Inet=0

Thus, no current flows through an unbiased diode.

Now let’s discuss the working when the diode is FORWARD BIASED.

The diode is in forward bias mode when the positive and negative terminals of a DC voltage source is connected to p-side and n-side of the diode respectively.

Also Read: Zener Diode, Its Working, Zener Breakdown And Its Applications

This increases the potential at the p-side and decreases the potential at the n-side of the diode. This potential balances the barrier potential which is generated internally in the diode. Thus, the diffusion of electrons and holes is thereby increased and current flows through the semiconductor.

pn junction diode

As the barrier potential decreases due to the external potential, the depletion layer also becomes smaller.

Therefore when the diode is forward biased,we can conclude that,

Idf>IdrIdf>Idr

and also that the net current flows from p-side to n-side.

The working of REVERSE BIASED junction diode is exactly opposite of that of forward biased.

The diode is in reverse bias mode when the positive and negative terminals of a DC voltage source is connected to n-side and p-side of the diode respectively.

pn junction diode

This increases the potential at the n-side and decreases the potential at the p-side of the diode. This potential drop is in the same direction as that of barrier potential drop. Therefore, the diffusion of electrons and holes is almost stopped and thus no current flows through the semiconductor.

Also Read: Zener Diode, Its Working, Zener Breakdown And Its Applications

As the barrier potential increases due to the external potential, the depletion layer also becomes wider.

Therefore when the diode is reverse biased,we can conclude that the net current flows from n-side to p-side as

Idr>IdfIdr>Idf

pn junction diode

Uses Of PN Junction Diode-

It is used in clipping circuits as wave shaping circuits in computers, radios, radars etc.
• It is used as switches in digital logic designs.
• It is used in detector and demodulator circuits.
• It is used in clamping circuits in TV receivers as well as voltage multipliers.
• It is used as rectifiers in DC power supply manufacturing.

Also Read: Zener Diode, Its Working, Zener Breakdown And Its Applications

The device having same P-N material but different doping concentrations is known as homojunction. This will have many applications as mentioned below.
• Used as zener diode in peak clippers, voltage regulators and for switching.
• Used as varactor diode for tuning applications in TV and radio receivers.
• Used as PIN photodiode in an optical communication system.
• Used as tunnel diode as an oscillator at microwave frequencies.
• Used as Laser diodes for optical communication.
• Used as LEDs in various display panels.

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